For WFC3, zeropoints represent the flux or magnitude corresponding to a (flat-field corrected) count rate of 1 electron per second, and are used to convert instrumental to astrophysical units.
The tables below are the derived quantities from observations of GD153, G191B2B, GD71, and P330E acquired between 2009 and 2012. The independent calibrations from the four stars agree to within 1% in most filters, and the photometric zeropoint is set to the average of the measurements. The data sets were processed with AstroDrizzle.
The methodology used to determine the zeropoints will be described in a forthcoming ISR.
Zeropoint tables after March 06, 2012:
The methodology used to determine the zeropoints will be described in a forthcoming ISR.
Zeropoint tables after March 06, 2012:
For an infinite aperture:
IR
For a 0.4 arcsec radius aperture:
IR
Zeropoint tables prior to March 06, 2012:
IRAperture Corrections and Encircled Energy Curves
Aperture Corrections and EE curves for WFC3/IRPhotometric Systems:
- VEGAmag : SYNPHOT standard magnitude system, for which Vega by definition has magnitude 0 at all wavelengths. The vega magnitude of a star with flux F is -2.5 log10 (F/F_vega) where F_vega is the calibrated spectrum of Vega in Synphot.
- STmag and ABmag:
both systems define an equivalent flux
density for a source, corresponding to the flux density of a source
of predefined spectral shape that would produce the observed count
rate, and convert this equivalent flux to a magnitude. The
conversion is chosen so that the magnitude in V corresponds roughly
to that in the Johnson system. In the STmag system, the flux density
is expressed per unit wavelength, and the reference spectrum is flat
in F_lam , while in the ABmag system, the flux density is expressed
per unit frequency , and the reference spectrum is flat in F_nu.
The definitions are:
- STmag = -2.5 Log F_lam -21.10
- ABmag = -2.5 Log F_nu - 48.6
Photometric Keywords in the SCI extention of WFC3 images:
(keywords affected by the sensitivity curve update are highlighted)- PHOTMODE: Observation configuration for photometric calibration.
- PHOTFLAM: inverse sensitivity (erg cm^{-2} s^{-1} Ang^{-1}).
- PHOTZPT: ST magnitude zeropopint (= 21.10).
- PHOTPLAM: pivot wavelength.
- STMAG_ZEROPOINT = -2.5 Log (PHOTFLAM) - PHOTZPT = -2.5 Log (PHOTFLAM) - 21.10
- ABMAG_ZEROPOINT = -2.5 Log (PHOTFLAM) - 21.10 - 5 Log (PHOTPLAM) + 18.6921
Users performing photometry on non-drizzled _flt images may wish to read the description of the pixel area map.
Created 09/09/2009 MJDModified 03/06/2012 by CMP